Fine-tune SVG icons per UI team recommendations

@cleercode, this is a follow-up to #2551. I've adjusted the size of all the buttons to 24px and made the disclose triangles 1px higher. I didn't change the size of the tab close buttons, though: I wasn't sure if you'd meant to change them too, and 24px for them looked pretty excessive, competing with the main buttons. Please advise.

![screen shot 2014-06-16 at 9 26 43 pm](https://cloud.githubusercontent.com/assets/5606182/3296306/c7633dc6-f5d7-11e3-8f31-0c10d62f70ee.png)